Comminution is the transformation of particles of a hard material into a greater number of smaller size particles and can include any of the processes of crushing, grinding or milling.
A roll crusher is an apparatus which may be utilized in the cement and in other mineral processing industries for carrying out the comminution of brittle material. Roll crushers consist of one or two rolls which function by crushing particles of brittle material between the rotating rolls or between one roll and a stationary breaker plate.
In pressure comminution of brittle grinding stock in the roller gap of a roll crusher, the grinding stock is crushed or comminuted by means of compressive stress. The feed for the roll crusher may be uniformly distributed over the width of the roll(s). Alternatively, the feed may be distributed to the roll by a method known as "choke feeding", wherein the feed is fed to the roll in sufficient volume to produce a packed or compact mass of material as it passes between the rolls so that the particles of the feed mutually crush one another in the roller gap to produce an agglomerated product bed. The product which is comminuted by a roll crusher is generally conveyed, after a deagglomeration step, if necessary, to a classifier and, typically, product exceeding a certain size is recirculated back through the roll crusher where it is intermingled with fresh feed which is being delivered into the roll crusher. Obviously, there is a physical limitation as to how much of the product can be recirculated back through the roll crusher with fresh feed.
When a high degree of comminution or high fineness of the grinding stock is to be achieved, multi-stage grinding methods such as pre-comminution, mean comminution and fine comminution have been employed. The multi-stage grinding methods which can, for example, consist of comminution machines such as roller mills, roll crushers and/or ball mills connected in series, however, produce a high overall specific energy consumption as well as high overall capital costs. Ball mills are distinguished by high degree of comminution and can grind chunks of cement clinker to cement fineness but the specific energy required by ball mills, however, is high.
U.S. Pat. No. 4,357,287, Schonert, teaches a method of carrying out the fine and very fine comminution of brittle material wherein a bulk of brittle material is stressed once between two practically non-yielding hard surfaces, such as the rolls of a roll crusher, with a compression of at least 500 kg/cm.sup.2 to result in energy sufficiently high to cause comminution and to also cause a distinct agglomeration or briquetting of the particles, and where the resulting agglomerates or briquettes are disintegrated by further mechanical stressing in a separate device. This referenced patent teaches and claims that by carrying out such stressing in a single pass the energy needed to comminute said particles will be substantially reduced. This reference does not teach or suggest that multiple stressing as characterized by more than one pass of the material through the roll crusher would result in energy savings.
It is now been surprisingly discovered that if the material to be stressed is subjected to at least two separate passes through the same roll crusher wherein the recirculated material is not co-mingled with fresh feed, at pressures which for both stresses is sufficiently high to cause the formation of at least some agglomerates on both passes, there is realized substantial energy savings over the process as taught in U.S. Pat. No. 4,357,287. This energy savings is completely unexpected in view of the teaching of said patent. It has also been discovered that a continuous process for such multiple stressing can be carried out on a novel roll crusher which has distinct initial and secondary inlet, crushing and outlet zones, whereby any material that is present in the initial inlet or crushing zone is routed so as to minimize intermingling with material that is present in the secondary inlet or crushing zone.